Optical cavity for gas sensor and gas sensor having optical cavity

Provided are an optical cavity 100 for a gas sensor which has a space therein and a gas sensor using the optical cavity, in which in the space of the optical cavity 100, an elliptical reflective surface 133, which constitutes a part of an ellipse (133, 133a) and reflects the light emitted from a position of one focal point F1 of the ellipse to concentrate the light on the other focal point F2 of the ellipse, is formed, a hyperbolic reflective surface, which constitutes a part of a hyperbola (135a, 135b) having one focal point that coincides with the other focal point of the ellipse, and reflects the light, which is reflected by the elliptical reflective surface and concentrated on the other focal point of the ellipse, to concentrate the light on the other focal point f2 of the hyperbola, is formed, and a hyperbola centerline B-B′, which connects one focal point of the hyperbola and the other focal point of the hyperbola, is inclined toward a side opposite to the elliptical reflective surface by a predetermined angle with respect to an ellipse centerline A-A′ which connects one focal point of the ellipse and the other focal point of the ellipse.